Hollow capsules have been widely used in industries including pharmacy, food, cosmetics and so on. It has been extensively developed and applied.
Most of hollow capsules nowadays are fabricated using gelatin as main molding gel. However, a lot quality defects are shown during the application process due to the characteristics of gelatin itself. For example, gelatin molecules may generate a tough, flexible and water insoluble membrane through self-oxidation reaction or intramolecular and intermolecular cross-linking reactions with functional groups such as aldehyde group. This membrane may hinder the drug release and it will result in the disqualification of capsule disintegration. Besides, when using gelatin as main molding gel, it is prone to appear quality defects, such as oil leakage, adhesion and further oxidation of the oxidizable component in long time storage. Moreover, with the increase of religion and vegetarianism, as well as the successive appearance of BSE (bovine spongiform encephalopathy), foot-and-mouth disease events all over the word, people begin to feel unsafe about the gel made from bones and skin of cows and other animals. So it is very necessary to develop non-animal capsule shells. At the present stage, there are three kinds of widely investigated non-animal capsule shells. The first one is mainly made of water soluble cellulose derivatives, such as HPMC (hydroxypropyl methylcellulose) hollow capsules, but it has high oxygen permeability, poor disintegrating stability and appearance properties. The second one is mainly made of starch. However, it has poor friability and cannot hold liquids. The third one is mainly made of polysaccharide polymers, among which hollow capsules made of pullulan polysaccharide are mostly studied. This kind of capsule has low oxygen permeability, good transparency and short disintegration time, thus it becomes a research hotspot.
Domestic and foreign experts and scholars provide a lot of methods to improve the quality of capsules containing pullulan polysaccharide. For example, it is proposed in U.S. patent Ser. No. 10/941,182 that pullulan polysaccharide hard or soft capsules can be made using pullulan polysaccharide as main ingredient and surfactants, gels, cationic coagulant aids and chelating agents as additives. Chinese patent NO. CN101069677 shows that hollow pullulan polysaccharide capsules can be made using pullulan polysaccharide as main ingredient and gels, cationic coagulant aids, surfactants and moisturizing agents as additives. Chinese patent NO. CN201110420285.0 also proposed that hollow pullulan polysaccharide capsules can be made using the pullulan polysaccharide as main ingredient and hydrophilic gels, plasticizer, surfactants and moisturizing agents as additives.
Although the methods proposed above have some effects on improving the quality of pullulan polysaccharide capsules, there are still some shortages of pullulan polysaccharide capsules in practical use. For example, the pullulan polysaccharide capsules have poor storage stability. It will dehydrate and get brittle and dark after long time storage. The requirement of the humidity of storage environment is very strict. When the humidity is slightly higher, the capsule will turn softer. What's more, alkali metal ions are introduced into the preparation process of pullulan polysaccharides capsules, which will influence the disintegration of the capsules. So, further improvement is needed.